The present invention relates to combustors and, more particularly, to cavity staging in a combustor.
A conventional gas turbine engine typically includes a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating a high pressure, high temperature gas stream, referred to as a post combustion gas. The post combustion gases flow to a turbine, where they are expanded, converting thermal energy from the post combustion gases to mechanical energy for driving a shaft to power the compressor and produce output power for powering an electrical generator, for example.
Electrical power generating gas turbine engines are typically operated for extended periods of time and exhaust emissions from the combustion gases are a concern and are subject to mandated limits. Thus, combustors are preferably designed for low exhaust emissions operation and, in particular, for low NOx operation. Typical designs for the combustor may include can, cannular or annular configurations.
In an exemplary embodiment, a typical low NOx can combustor includes a plurality of combustor cans circumferentially adjoining each other around the circumference of the engine. Can-annular combustors typically consist of a cylindrical can-type liner inserted into a transition piece. Annular combustors are also used in many gas turbine applications and include multiple pre-mixers positioned in rings directly upstream of the turbine nozzles in an annular fashion. An annular burner has an annular cross-section combustion chamber bounded radially by inner and outer liners while a can burner has a circular cross-section combustion chamber bounded radially by a single liner.
Irrespective of the combustor configuration, a typical low NOx combustor includes a premixer, including a swirler, having a plurality of circumferentially spaced apart vanes disposed at a duct inlet for swirling compressed air received from the engine compressor. In a can combustor configuration, each combustor can has a plurality of pre-mixers joined to the upstream end. In a can-annular combustor configuration, multiple fuel-air pre-mixers are positioned at the head end of the cylindrical can-type liner. Disposed downstream of the swirler(s) are suitable fuel injectors typically in the form of a row of circumferentially spaced-apart fuel spokes, each having a plurality of radially spaced apart fuel injection orifices which conventionally receive fuel, such as gaseous methane, through the centerbody for discharge into the pre-mixer duct upstream of a combustor dome.
Typically fuel injectors are disposed axially upstream from the combustion chamber so that the fuel and air have sufficient time to mix, typically by way of a pre-mixer, and pre-vaporize. In this way, the pre-mixed and pre-vaporized fuel and air mixture support cleaner combustion thereof in the combustion chamber for reducing exhaust emissions.
In an attempt to minimize the overall weight of engine systems, another type of combustor, referred to herein as an ultra-compact combustor (UCC), comprises a combustor having integrated as a part thereof, turning vanes of the turbine. The integration of the turning vanes and the combustor provides for the length of the combustor, and thus the overall system, to be reduced. The combustor typically includes one or more circumferential cavities in which fuel and air are injected and combustion happens in a trapped vortex. Combustion products pass from the cavity into the main bulk flow and occur in a rich-quench-lean manner.
Benefits of the use of ultra-compact combustors include reduced combustor size, weight reduction, and fewer parts. Challenges encountered during the use of ultra-compact combustors include managing pressure losses and achieving as complete combustion as possible in as small residence time as possible, and in turn minimize NOx and CO emissions.
Accordingly, there exists a need in the art to provide for a combustor that improves fuel-air mixing and that gradually introduces combustion products into the main flow, thereby providing for complete combustion and reduced NOx and CO emissions.